Direct techniques will be used in this study to quantitatively define the mechanical states of microcirculatory vessels and to examine the extent to which the mechanical states act as a determinant of regional microvascular response differences to physical, chemical and neural stimuli. The mechanical states of microvessels will be defined in terms of initial tangential stress-diameter relationships. The data for calculating the in vivo stress-diameter states will be obtained by direct, simultaneous measurements of pressure, diameter, and wall thickness in single microvessels. Gross pressure-flow parameters will also be controlled and measured. After defining the control states of microvessels, we will define the mechanical conditions extant in separate microvascular segments which are required in order for neural, local chemical, and physical stimuli to elicit maximum vascular responses. In addition, studies will be conducted to examine the ways in which the mechanical states of microcirculatory vessels interact to alter capillary hydrostatic pressure. Capillary pressures will be measured with the servo-null system. Particular attention will be given to direct, comparative studies in red and white skeletal muscle microvessels in order to distinguish functional differences in microvascular reactivity, capillary hydrostatic pressure, and transcapillary filtration characteristics within these two muscle types. These studies will help discern the extent to which capillary hydrostatic pressure may be regulated, and will help answer questions which cannot be resolved by the application of indirect techniques.